Reviewer #1: ...importance of genetic factors in formation ofAVMs has never been established... This important point is now carefully addressed in Section 2.3. Although most AVMs are sporadic, there is indeed evidence supporting a familial component. A recent review article identified 53 cases of AVM without Hereditary Hemorrhagic Telangectasia (HHT) in 25 families.39 Further, there has been one linkage and association analysis, which identified seven candidate regions, with the strongest signal at chromosome 6q25 (LOD=1.88;P=0.002).25 This study was unfortunately underpowered due to the small sample size. Further evidence of a genetic component to sporadic AVMs comes from estimation of sibling recurrence risk ratio of disease, an important consideration raised by Reviewer #1. This measure, Sibling, is defined as the risk of disease in siblings of an individual with disease, divided by the population prevalence of the disease.40 Complex diseases display a wide range of Xsibiing values.41'43 Based on the reported literature, we present in Section 2.3 estimates for XSibiing in AVM which are consistent with a genetic contribution to the disease. Taken together, then, there is at least modest evidence supporting familial aggregation for the AVM phenotype, although definitive proof is lacking. Perhaps use of the term "sporadic" also reinforces an unwarranted assumption that no genetic underpinning exists. However, in context of our preliminary and other published data, it seems reasonable to proceed on the notion that genetic variation at least influences the susceptibility and clinical course of the disease. We do not hypothesize that "formation" of AVM lesions is caused by a single genetic locus. Rather, AVM is a complex disease that is likely due to many factors. Even Hereditary Hemorrhagic Telangectasia almost certainly requires some as yet unknown set of modifier genes or factors for the AVM phenotype to be expressed. Variation in such modifier genes may be heritable, e.g., there are multiple genetic loci that appear to control VEGF-induced angle-genesis.60 61 if...AVMs are sporadic, the investigators ... need to [be] searching for somatic genomic variants. Although we believe there is ample evidence for a genetic component in AVM susceptibility, somatic mutation is an excellent suggestion for an alternative mechanism. There is evidence for a "second-hit" model in cavernous malformations6'64 and systemic venous malformations.2'3 As exciting as this hypothesis is, it also presents significant logistical problems for AVMs since we do not know what gene to target, although ENG and ALK-1 might be reasonable candidates. Nonetheless, we agree that it is scientifically important to initiate investigation of somatic genetic variation in AVMs. Therefore, we have made the search for somatic mutations an exploratory aim, adding Doug Marchuk as a Co-investigator, an expert in somatic mutation studies. Candidate gene methodology...given [its] track record ...in complex medical disorders ...is not welljustified. Many of the reviewer's points in this regard are well taken. Accordingly, we have now reshaped the approach to focus on genome-wide association (GWAS). However, we can still leverage our knowledge of plausible candidates in consideration of how we follow up our initial GWAS analyses (see Section 4.8.1.A). The reviewer brought up the important issue of replication. We now emphasize (Section 3.3.1) that we recently provided the first description of a common genetic variant associated with sporadic AVMs: an intronic variant of ALK-1 that was present at a higher frequency in AVM cases compared to healthy controls.81 This association was independently replicated by the group at the University of Bonn. Germany;78 a meta-analysis of combined results was published.79'146 We now provide new preliminary data that this single nucleotide polymorphism (SNP) is associated with alternative splicing (Section 3.3.1), which sets the stage for functional characterization of the effects of this variant on the gene product. Data for candidate genetic variants that predict spontaneous hemorrhage, however, have not been replicated; these results are presented as preliminary data. A longitudinal cohort with an infrequent outcome event (hemorrhage) is far more difficult to obtain with so few research groups active in this area. We would suggest that for a rare disease like AVM, it is not only reasonable but highly constructive to report initial associations of important clinical associations, despite the well-known propensity for false positives. The ALK-1 SNP example given above is a good example of such synergism between different research groups. PHS 398/2590 (Rev.11 /07) Pane 171